I have raised and shown Danes. As is stated by others here, you do not feed Dane puppies puppy food. However you will need to switch over a few days time.
The most important thing is to find a veterinarian that specializes in large/giant breeds. They are not the same and they need to be treated differently.
Ask the breeder how long his/her dogs live? how old their oldest dog is?. Might give you an insight with the bloodline. Ask if there is any bloat or hip displaysia in the line.
My experience with the breed is that they are giant destructive baby children until they turn 2 and then they are magically total angels. I would not trade one minute of the hard/sad times with these special beasts. I loved them very very very much.
And yes, they want a spin or 3 around the yard and the rest of their day is spent sitting on your lap.
Best Wishes!!!! you are in for a grand time with a grand breed.